1. Technical Field
The invention relates in general to a code division communication system and associated method, and more particularly to a downlink orthogonal variable spreading factor (OVSF) code assignment method and a base station system using the same.
2. Description of the Related Art
Orthogonal variable spreading factor (OVSF) codes are codes indicating a construction based on a binary tree. An OVSF code tree, hereinafter, code tree, is a full binary tree. Referring to FIG. 1, each node of the code tree can be uniquely determined according to a spreading factor and a layer sequence number. For example, C2,1 represents the first node of SF2 layer, and so forth.
In a Wideband Code Division Multiple Access (WCDMA) system for Universal Mobile Telecommunications System (UMTS), a single OVSF code is assigned for a connection request, a connection of each mobile terminal is identified through orthogonality of the OVSF codes, wherein the OSVF codes serve as downlink channelization codes.
Due to the orthogonality constraint on OVSF code assignment, after an OVSF code is assigned, another OVSF code can be assigned only if the latter is of the same layer as that of the former or is not an ancestor or descendent node of the former, i.e., the former is orthogonal to the latter. In other words, a node and its ancestor node or descendent node are not orthogonal and cannot be simultaneously assigned. Such issue of unusable nodes resulted from the orthogonality constraint is referred to as code blocking.
An OVSF code assignment method affects whether code resource can be utilized sufficiently. Current code assignment methods are faced with certain difficulties of producing code fragments. For example, a leftmost code assignment method, which can be easily practiced but its algorithm has lower efficiency, renders a higher code blocking rate for high-speed service requests and produces a quite large number of fragments during service release. A leftmost fragment assignment method can also be easily practiced, and offers better efficiency of code fragment reduction than that of the leftmost code assignment. However, it still fails to solve the code fragment issue produced during service release. Further, a weighting method is more complicated to implement, and has a higher efficiency of code fragment reduction as well as a lower code blocking rate than the two previous methods. Yet, the weighting method is also incapable of solving the code fragment issue produced during service release.
Code management is targeted at management of a downlink OVSF code tree, which is utilized to assign orthogonal physical channels to different users. Since available codes are limited, there is a need for a method for effectively assigning channel codes to prevent code blocking.